> how slowly do the other chickens grow?

Here in Norway the two largest producers are moving[1][2] from Ross 308 to Rustic Gold[3]. They live roughly 40-50% longer. The third largest has already moved[4] to the hybrid Hubbard JA 878, with similar growth.

However it's also easier to get the slow-growing chickens larger without health issues, roughly 25% according to this[5], so it turns out you might need fewer chickens to produce the same amount of meat. And thus the economics might not be as bad as it looks at first glance.

That's for Norway though, perhaps it's different elsewhere.

[1]: https://www.nortura.no/b%C3%A6rekraft-i-nortura/omsorg-for-m...

[2]: https://kommunikasjon.ntb.no/pressemelding/18709900/coop-tra...

[3]: https://aviagen.com/eu/brands/rowan-range/products/rustic-go...

[4]: https://dyrevern.no/landbruksdyr/kraftig-velferdsloft-for-no...

[5]: https://www.norsklandbruk.no/den-vanskelige-okonomien-i-a-by...